While the butterflies form a monophyletic group, the moths, which comprise the rest of the Lepidoptera, do not. Many attempts have been made to group the superfamilies of the Lepidoptera into natural groups, most of which fail because one of the two groups is not monophyletic: Microlepidotera and Macrolepidoptera, Heterocera and Rhopalocera, Jugatae and Frenatae, Monotrysia and Ditrysia.[2]

Although the rules for distinguishing moths from butterflies are not completely hard and fast, one very good guiding principle is that butterflies have thin antennae and (with one exception) have small balls or clubs at the end of their antennae. Moth antennae can be quite varied in appearance, but in particular lack the club end. The divisions are named by this principle: "club-antennae" (Rhopalocera) or "varied-antennae" (Heterocera).

The modern English word "moth" comes from Old English "moððe" (cf. Northumbrian "mohðe") from Common Germanic (compare Old Norse "motti", Dutch "mot", and German "motte" all meaning "moth"). Its origins are possibly related to the Old English "maða" meaning "maggot" or from the root of "midge" which until the sixteenth century was used mostly to indicate the larva, usually in reference to devouring clothes.

The study of butterflies and moths is known as "lepidoptery", and biologists who specialize in either are called "lepidopterists". As a pastime, watching butterflies and moths is known as "butterflying" and "mothing". The latter has given rise to the term "mother" for someone who engages in this activity - sometimes written with a hyphen (moth-er) to distinguish it from the more common word of the same spelling. This confusion does not arise in speech as it is pronounced differently (/ˈmɒθər/, not */ˈmʌðər/).

Moth larvae, or caterpillars, make cocoons from which they emerge as fully grown moths with wings. Some moth caterpillars dig holes in the ground, where they live until they are ready to turn into adult moths.[3]

Moths evolved long before butterflies, fossils having been found that may be 190 million years old. Both types of lepidoptera are thought to have evolved with flowering plants, mainly because most modern species feed on flowering plants, as adults and larvae. One of the earliest species thought to be a moth-ancestor is archaeolepis mane, whose fossil fragments show scaled wings similar to caddisflies in their veining.[4]

An adult male pine processionary moth (Thaumetopoea pityocampa). This species is a serious forest pest when in larval state. Notice the bristle springing from the underside of the hindwing (frenulum) and running forward to be held in a small catch of the forewing, whose function is to link the wings together.

Several moths in the family Tineidae are commonly regarded as pests because their larvae eat fabric such as clothes and blankets made from natural proteinaceous fibers such as wool or silk.[6] They are less likely to eat mixed materials containing some artificial fibers. There are some reports that they may be repelled by the scent of wood from juniper and cedar, by lavender, or by other natural oils; however, many consider this unlikely to prevent infestation. Naphthalene (the chemical used in mothballs) is considered more effective, but there are concerns over its effects on human health.

Moth larvae may be killed by freezing the items which they infest for several days at a temperature below −8 °C (18 °F).[7]

Some moths are farmed. The most notable of these is the silkworm, the larva of the domesticated moth Bombyx mori. It is farmed for the silk with which it builds its cocoon. As of 2002[update], the silk industry produces more than 130 million kilograms of raw silk, worth about 250 million U.S. dollars, each year.[8][9][10]

The mopane worm, the caterpillar of Gonimbrasia belina, from the family Saturniidae, is a significant food resource in southern Africa. Despite being notorious for eating clothing, most moth adults do not eat at all. Many, like the Luna, Polyphemus, Atlas, Prometheus, Cecropia, and other large moths do not have mouths. Among those with adults that do eat, they will drink nectar.[6]

Nocturnal insectivores often feed on moths; these include some bats, some species of owls and other species of birds. Moths also are eaten by some species of lizards, cats, dogs, rodents, and some bears. Moth larvae are vulnerable to being parasitized by Ichneumonidae.

Baculoviruses are parasite double-stranded DNAinsect viruses that are used mostly as biological control agents. They are members of the Baculoviridae, a family that is restricted to insects. Most baculovirus isolates have been obtained from insects, in particular from Lepidoptera.

There is evidence that ultrasound in the range emitted by bats causes flying moths to make evasive maneuvers because bats eat moths. Ultrasonic frequencies trigger a reflex action in the noctuid moth that causes it to drop a few inches in its flight to evade attack.[11]Tiger moths also emit clicks which foil bats' echolocation.[12][13]

Moths frequently appear to circle artificial lights, although the reason for this behavior remains unknown. One hypothesis to explain this behavior is that moths use a technique of celestial navigation called transverse orientation. By maintaining a constant angular relationship to a bright celestial light, such as the moon, they can fly in a straight line. Celestial objects are so far away, that even after travelling great distances, the change in angle between the moth and the light source is negligible; further, the moon will always be in the upper part of the visual field, or on the horizon. When a moth encounters a much closer artificial light and uses it for navigation, the angle changes noticeably after only a short distance, in addition to being often below the horizon. The moth instinctively attempts to correct by turning toward the light, causing airborne moths to come plummeting downward, and resulting in a spiral flight path that gets closer and closer to the light source.[14]

^Chihuahuan Desert Nature CenterEvolution of Moths and Butterflies
Studying the evolution of butterflies and moths is challenging, since fossils are so rare. But the few Lepidopteran fossils that exist, captured in amber or compressed in fine-grained rocks, show an astonishing amount of detail. The earliest Lepidopteran fossils appear in rocks that are about 190 million years old. These tiny fragments of scaled wings and bodies clearly indicate that moths evolved before butterflies.